\psset{unit=1.5, ticksize=-3pt 0, algebraic=true}
\begin{pspicture*}(-1,-0.5)(3.5,2.5)
\footnotesize
\psaxes[arrows=<->,ticks=none,labels=none](0,0)(-0.5,-0.5)(3.5,2.5)
\psplot[linecolor=blue]{0.5}{3.1}{(x-1.8)^2+0.5}
\psline[linecolor=red,linestyle=dashed](0.9,0)(0.9,1.31)(0,1.31)
\psline[linecolor=red,linestyle=dashed](2.9,0)(2.9,1.71)(0,1.71)
\psline[linecolor=green,arrows=<->](0.9,1.31)(2.9,1.71)
\psxTick(0.9){x_1}
\psxTick(2.9){x_2}
\psyTick(1.31){f(x_1)}
\psyTick(1.71){f(x_2)}
\end{pspicture*}